1. Field of the Invention
The present invention relates to an electrophotographic light-receiving member having a sensitivity to electromagnetic waves such as light (which herein refers to light in a broad sense and indicates ultraviolet rays, visible rays, infrared rays, X-rays, xcex3-rays, etc.), and also relates to a process for its production.
2. Description of the Related Art
In the field of image formation, photoconductive materials that form light-receiving layers in light-receiving members are required to have properties such that they are highly sensitive, have a high SN ratio [light current (Ip)/dark current (Id)], have absorption spectra suited to spectral characteristics of electromagnetic waves to be radiated, have a high response to light, have the desired dark resistance and are harmless to human bodies when used. In particular, in the case of electrophotographic light-receiving members set in electrophotographic apparatus used in offices, their safety during use an important point.
Photoconductive materials having good properties in these respects include amorphous silicon hydrides (hereinafter xe2x80x9ca-Si:Hxe2x80x9d). For example, U.S. Pat. No. 4,265,991 discloses its application in electrophotographic light-receiving members. In such electrophotographic light-receiving members having a-Si:H, it is common to form photoconductive layers comprised of a-Si, by film forming processes such as vacuum deposition, sputtering, ion plating, heat-assisted CVD, light-assisted CVD and plasma-assisted CVD while heating conductive to from 50xc2x0 C. to 350xc2x0 C. In particular, the plasma-assisted CVD, i.e., a process in which material gases are decomposed by direct-current, high-frequency or microwave glow discharging to form a-Si deposited films on the support, has been put into practical use as a preferred process.
U.S. Pat. No. 5,382,487 discloses an electrophotographic light-receiving member comprising a conductive support and an amorphous silicon photoconductive layer containing a halogen atom as a constituent (hereinafter xe2x80x9ca-Si:Xxe2x80x9d photoconductive layer). This publication discloses that incorporation of 1 to 40 atom % of halogen atoms into a-Si enables achievement of a high thermal resistance, and also electrical and optical properties preferable for a photoconductive layer of an electrophotographic light-receiving member.
Japanese Patent Application Laid-Open No. 57-115556 also discloses a technique in which a surface barrier layer formed of a non-photoconductive amorphous material containing silicon atoms and carbon atoms is provided on a photoconductive layer formed of an amorphous material mainly composed of silicon atoms, in order to achieve improvements in photoconductive members having a photoconductive layer formed of an a-Si deposited film, in respect of their electrical, optical and photoconductive properties such as dark resistance, photosensitivity and response to light and service environmental properties such as moisture resistance and also in respect of stability with time. Japanese Patent Publication Laid-Open No. 60-67951 also discloses a technique concerning a photosensitive member laminated with a light-transmitting insulating overcoat layer containing amorphous silicon, carbon, oxygen and fluorine. U.S. Pat. No. 4,788,120 still also discloses a technique in which an amorphous material containing silicon atoms, carbon atoms and 41 to 70 atom % of hydrogen atoms as constituents is used to form a surface layer.
U.S. Pat. No. 4,409,311 further discloses that a highly sensitive and highly resistant, electrophotographic photosensitive member can be obtained by using in a photoconductive layer an a-Si:H containing 10 to 40 atom % of hydrogen atoms and having absorption peaks at 2,100 cmxe2x88x921 and 2,000 cmxe2x88x921 in an infrared absorption spectrum which peaks are in a ratio of 0.1 to 1.7 as the coefficient of absorption. Japanese Patent Publication Laid-Open No. 62-83470 further discloses a technique in which a high quality image without after image can be obtained by reducing, to 0.09 eV or less, the characteristic energy of the exponential tail in a light absorption spectrum of a photoconductive layer of an electrophotographic photosensitive member.
Meanwhile, U.S. Pat. No. 4,607,936 discloses a technique in which, aiming at an improvement in image quality of an amorphous silicon photosensitive member, image forming steps such as charging, exposure, development and transfer are carried out while maintaining temperature at 30 to 40xc2x0 C. in the vicinity of the surface of the photosensitive member to thereby prevent the surface of the photosensitive member from undergoing a decrease in surface resistance which is due to water absorption on that surface and also smeared images from occurring concurrently therewith.
These techniques have achieved improvements in electrical, optical and photoconductive properties and service environmental properties of electrophotographic light-receiving members, and also have concurrently brought about an improvement in image quality.
The electrophotographic light-receiving members having a photoconductive layer comprising an a-Si material have individually achieved improvements in properties in respect of electrical, optical and photoconductive properties such as dark resistance, photosensitivity and response to light and service environmental properties and also in respect of stability with time, and durability. Under existing circumstances however, there is room for further improvements to make overall properties better. In particular, there is a rapid progress in making electrophotographic apparatus have higher image quality, higher speed and higher durability, and the electrophotographic light-receiving members are required to be more improved in electrical properties and photoconductive properties and also to be significantly decreased in variations in their properties in every environment while being improved in charge performance and sensitivity.
Then, as a result of improvements made on optical exposure devices, developing devices, transfer devices and the like in order to improve image characteristics of electrophotographic apparatus, the electrophotographic light-receiving members are now also required to be more improved in performance than ever.
Under such circumstances, although the conventional techniques as described above have made it possible to improve properties to a certain degree in respect of the above-described subjects, they cannot be said to be satisfactory in regard to additional enhancement in charge performance and image quality. In particular, as the subjects for making amorphous silicon light-receiving members have much higher image quality, it has now been sought to further improve the charge performance and decrease variations in the charge performance due to changes in environmental temperature and exposure memory such as blank memory and ghost. For example, an electrophotographic apparatus is miniaturized year by year from the viewpoint of space saving, and the space around a photosensitive member in the electrophotographic apparatus tends to be decreased concurrently. As a result, although a charger is miniaturized, it becomes difficult to increase the capacity of a power source for the charger from the viewpoint of power consumption, thereby causing difficulties in ensuring a sufficient surface potential.
For example, hitherto, in order to prevent smeared images caused by photosensitive members, a drum heater is set inside a copying machine to keep the surface temperature of a photosensitive member at about 40xc2x0 C., as disclosed in Japanese Patent Publication Laid-Open No. 60-95551. In conventional photosensitive members, however, the dependence of charge performance on temperature, so-called temperature-dependent properties, which is ascribable to formation of pre-exposure carriers or heat-energized carriers is so great that, in the actual service environment inside copying machines, photosensitive members were sometimes used with lower charge performance than that originally possessed by the photosensitive members. For example, the charge performance may drop by nearly 100 V in the state where the photosensitive members are heated to about 40xc2x0 C. by a drum heater, compared with the case when used at room temperature.
At night when copying machines are not used, the drum heater is kept electrified in conventional cases so as to prevent the smeared images that are caused when ozone products formed by corona discharging of the charger are adsorbed on the surface of a photosensitive member. Nowadays, however, it has become popular not to electrify copying machines at night for the purpose of saving natural resources and saving electric power. When copies are continuously taken in such a state, the surrounding temperature of the photosensitive member inside a copying machine gradually rises to make charge performance lower with a rise of the temperature of the photosensitive member, causing the problem of a change in image density during copying.
On the other hand, when the same original is repeatedly continuously copied, blank memory and so-called ghost phenomenon have now become problems to the further improvement of image quality; the blank memory being a phenomenon which causes a density difference on copied images, caused by the effects of so-called blank exposure that is applied to the photosensitive member in order to save the amount of the toner used, and the ghost being a phenomenon in which an image remaining after the image exposure in previous copying is produced on an image in the subsequent copying.
Accordingly, in designing electrophotographic light-receiving members, it is required to achieve improvements from the overall viewpoints of layer configuration and chemical composition of each layer of electrophotographic light-receiving members so that the problems as described above can be solved, and also to achieve a much more improvement in properties of the a-Si materials themselves.
The present invention aims at solving the problems involved in electrophotographic light-receiving members having the conventional light-receiving layer formed of a-Si as described above.
That is, a main object of the present invention is to provide an electrophotographic light-receiving member which is substantially always stable almost without impact on electrical, optical and photoconductive properties in service environments, has a superior resistance to exposure fatigue, has superior durability and moisture resistance without causing any deterioration when repeatedly used, can be almost free from residual potential and also can achieve a good image quality, and a process for its production.
Another object of the present invention is to provide an electrophotographic light-receiving member in which temperature characteristics and exposure memory are decreased while significantly improving charge performance and sensitivity, to significantly improve image quality, and a process for its production.
The present invention provides an electrophotographic light-receiving member comprising a conductive support and a light-receiving layer having a photoconductive layer formed on the surface of the conductive support and composed of a non-single crystal material containing silicon atoms as a main component and hydrogen atoms and/or halogen atoms; wherein the non-single crystal material, which constitutes the photoconductive layer, has an optical band gap of 1.8 eV to 1.85 eV, and the characteristic energy of exponential tail obtained from a light absorption spectrum thereof is 50 meV to 55 meV.
The present invention also provides a process for producing an electrophotographic light-receiving member comprising a conductive support and a light-receiving layer formed on the surface of the conductive support and having a photoconductive layer composed of a non-single crystal material containing silicon atoms as a main component and hydrogen and/or halogen atoms, the process comprising forming the photoconductive layer under conditions in which the flow rate (X) [sccm] of a gas for supplying Si and a discharge space volume (Z) [cm3] satisfy the following relation (A), and the flow rate (X) [sccm] of the Si supplying gas and the density (Y) [W/cm3] of power supplied to the discharge space satisfy the following relation (B):
3xc3x9710xe2x88x923xe2x89xa6X/Zxe2x89xa61xc3x9710xe2x88x922xe2x80x83xe2x80x83(A)
3xc3x9710xe2x88x924xe2x89xa6Y/Zxe2x89xa67xc3x9710xe2x88x924xe2x80x83xe2x80x83(B)
In order to solve the above problems, the inventors intensively researched the relations between the local distribution of amorphous silicon in a band gap and charge performance and exposure memory with attention to the optical band gap of a photoconductive layer and the behavior of carriers in the photoconductive layer. As a result, it was found that the objects of the present invention can be achieved by controlling the local distribution in the photoconductive layer while enlarging the optical band gap. Namely, it was found that, with regard to a light-receiving member having a photoconductive layer composed of a non-single crystal material containing silicon atoms as a main component and hydrogen atoms and/or halogen atoms, a light-receiving member manufactured based on a design for specifying the layer structure thereof exhibits excellent practical characteristics, and is superior to conventional light-receiving members in all points, particularly, excellent in characteristics as an electrophotographic light-receiving member.
The present invention has been achieved on the basis of the finding. The light-receiving member of the present invention has a photoconductive layer composed of a non-single crystal material containing silicon atoms as a main component and hydrogen and/or halogen atoms, wherein the hydrogen content, the optical band gap and the characteristic energy of exponential tail obtained from a light absorption spectrum of the photoconductive layer are controlled so as to improve charge performance and temperature-dependent properties and prevent the occurrence of exposure memory, thereby exhibiting good characteristics.
In the present invention, xe2x80x9cexponential tailxe2x80x9d represents a tail on the low-energy side of a light absorption spectrum, and xe2x80x9ccharacteristic energyxe2x80x9d means the gradient of the exponential tail.
This will be described in detail below with reference to FIG. 2.
FIG. 2 shows an example of a sub-gap light absorption spectrum of amorphous silicon in which photon energy hv is shown on the abscissa, and a logarithm of absorption coefficient xcex1 is shown on the ordinate. This spectrum is roughly divided into two parts including part B (exponential tail or Urbach tail) where the absorption coefficient xcex1 exponentially changes, i.e., linearly changes, with photon energy hv, and part A where absorption coefficient xcex1 shows low dependence on photon energy hv.
Part B corresponds to the light absorption caused by optical transition from the tail level on the valence band side to the conduction band in amorphous silicon. In part B, the exponential dependence of absorption coefficient xcex1 on hv is represented by the following equation:
xcex1=a0 exp(hv/Eu)
If both sides of this equation are transformed to logarithms, the following equation is obtained:
ln xcex1=(1/Eu)xc2x7hv+xcex11
wherein xcex11=ln xcex10.
Therefore, the inverse (1/Eu) of characteristic energy Eu represents the gradient of absorption coefficient xcex1 in part B. Since characteristic energy Eu corresponds to the characteristic energy of an exponential energy distribution in the tail level on the valence band side, a low value of Eu indicates a low tail level on the valence band side.
In a band gap of a-Si:H, there are generally a tail (bottom) level ascribable to a structural disorder of Sixe2x80x94Si bonds and a deep level ascribable to structural imperfections of Si unbonded arms (dangling bonds) or the like. These levels are known to act as capture and recombination centers of electrons and holes to cause deterioration in properties of devices.
As methods for measuring the state of localized levels in such a band gap, deep-level spectroscopy, isothermal volume transient spectroscopy, photothermal polarization spectroscopy, photoacoustic spectroscopy and the constant photocurrent method are commonly used. In particular, the constant photocurrent method (abbreviated to xe2x80x9cCPMxe2x80x9d hereinafter) is useful as a method for simply measuring sub-gap light absorption spectra on the basis of the localized levels of a-Si:H.
The inventors have investigated the correlation between an optical band gap (abbreviated to xe2x80x9cEgxe2x80x9d hereinafter) and characteristic energy (abbreviated to xe2x80x9cEuxe2x80x9d hereinafter) at the exponential tail (Urbach tail) obtained from the sub-band gap light absorption spectrum measured by CPM and properties of a photosensitive member under various conditions. As a result, the inventors found that the Eg and Eu closely relate to charge performance, temperature-dependent properties and exposure memory of an amorphous silicon photosensitive member, and thus have accomplished the present invention.
Namely, it is made apparent from experiment by the inventors that a photoconductive layer, in which the optical band gap is enlarged as much as possible, and the rate of the carriers captured by the localized levels is decreased, enables a decrease in temperature-dependent properties while significantly improving charge performance, and substantial removal of the occurrence of exposure memory.
The a-Si electrophotographic light-receiving member comprises a surface layer and a charge injection blocking layer provided for blocking the injection of charge from the surface and the support in order to ensure the charge performance. However, such layers are insufficient to obtain higher charge performance, and it is necessary to increase the resistance of the photoconductive layer itself. However, the inventors found from experiment that a simple increase in the resistance causes not only problems with respect to the residual potential and exposure memory but also improvement in the charge performance which is not as good as expected. Namely, an amorphous silicon photosensitive member is generally charged by a method in which free carriers are generated by pre-charge exposure (i.e., pre-exposure), and then swept out by an electrical field during charging to create a state where the carriers are exhausted, to increase the apparent resistance. If many localized levels are present, the free carriers are not rapidly swept out, and thus the charge performance cannot be improved. Therefore, it is necessary for further improving the charge performance to increase the resistance as well as decreasing the number of localized levels.
As the cause of a decrease in charge performance which occurs when the photosensitive member is heated by a drum heater or the like, it is considered that carriers thermally excited are led by electric fields formed at the time of charging to move toward the surface while repeating their capture in and release from the localized levels of band tails and deep localized levels in a band gap, and consequently cancel surface charge. Here, the carriers having reached the surface while passing through a charger barely influence the decrease in charge performance, but the carriers having been captured in the deep levels reach the surface after they have passed through the charger,and cancel the surface charge, and hence this is observed as temperature-dependent properties. The carriers thermally excited after they have passed through the charger also cancel the surface charge to cause a decrease in charge performance. Accordingly, in order to decrease the temperature-dependent properties, it is necessary to hinder the thermally excited carriers from being produced by enlarging the optical band gap, and at the same time to improve the mobility of carriers.
The exposure memory is also caused when the photo-carriers produced by blank exposure or image exposure are captured in the localized levels in a band gap and the carriers remain in the photoconductive layer. More specifically, among photo-carriers produced in a certain copying process, the carriers having remained in the photoconductive layer are swept out by the electric field formed by surface charge at the time of subsequent charging or thereafter, and the potential at the portions exposed to light become lower than other portions, so that a density difference occurs on an image. Hence, the mobility of carriers must be improved so that they can move through the photoconductive layer at one process of copying without allowing the photo-carriers to remain in the layer.
Thus, when Eu of the photoconductive layer is controlled (decreased) while Eg is increased as in the present invention, it is possible to effectively improve the ability to hinder charge injection and hinder the production of thermally excited carriers, as well as decreasing the rate of the carriers captured in the localized levels, thereby significantly improving the mobility of the carriers. As a result, the charge performance and the temperature-dependent properties in the service environmental range of the photosensitive member can be significantly improved, and at the same time, the occurrence of exposure memory can be prevented. Hence, the stability of the photosensitive member to service environment can be improved, and high-quality images having a sharp halftone and a high resolution can be stably obtained.
Moreover, in the present invention, the intensity ratio of absorption peaks ascribable to Sixe2x80x94H2 bonds and Sixe2x80x94H bonds is specified, thereby the in-plane mobility of carriers in the light-receiving member is made uniform, so that a fine density difference in a halftone image, i.e., coarseness, can be decreased.
Hence, the above-mentioned construction of the present invention can both improve the charge performance and decrease the temperature-dependent properties and exposure memory in a high degree, can solve all problems of the conventional techniques, and can form a light-receiving member having excellent electrical, optical and photoconductive properties, image quality, durability and service environmental properties.